---
title: "Orthorhombic system; monoclinic mineral examples"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:orthorhombic-system-monoclinic-mineral-examples"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/orthorhombic-system-monoclinic-mineral-examples
site: BCT Engineering
---

# Orthorhombic system; monoclinic mineral examples

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2065 Shrawan (old course), 3+2 marks
- Video: [The 7 Crystal Systems!](https://www.youtube.com/watch?v=11Ng8CJNE7Y) — Rock Talk
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/orthorhombic-system-monoclinic-mineral-examples
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the orthorhombic crystal system with a suitable diagram and give examples of three minerals belonging to the monoclinic system.

## Short answer

The orthorhombic system has three unequal axes all at 90°, with 3 planes, 3 diad axes, and a centre of symmetry. Three monoclinic minerals are gypsum, orthoclase feldspar, and mica.

## Answer

#### Orthorhombic system
The **orthorhombic system** has three crystallographic axes, mutually perpendicular but all of **different lengths**.

$$
a \ne b \ne c, \qquad \alpha = \beta = \gamma = 90^\circ
$$

```text
        c
        |
        |_____ b
       /
      a        a, b, c unequal, all 90 deg
```

Symmetry of the normal class: 3 planes of symmetry (parallel to the axes), 3 diad axes (along a, b, c) and 1 centre of symmetry. The crystals are prisms with rectangular or rhombic cross-section. Minerals: topaz, barite, olivine, sulphur, aragonite.

#### Three monoclinic minerals
The monoclinic system has $a \ne b \ne c$, with $\alpha = \gamma = 90^\circ$ and $\beta \ne 90^\circ$. Examples:
1. **Gypsum**
2. **Orthoclase feldspar**
3. **Mica** (muscovite or biotite). Hornblende and augite are also monoclinic.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Orthorhombic axes are:
   A. Three equal perpendicular axes
   B. Three mutually perpendicular axes of different lengths
   C. Two equal and one unequal, perpendicular
   D. Three unequal, inclined
   Answer: B. a not equal b not equal c, all 90 degrees. (Key point: Orthorhombic axes)

2. The normal class of orthorhombic has:
   A. 9 planes, 13 axes and 1 centre
   B. 1 plane, 1 axis and 1 centre
   C. 3 planes, 3 diad axes and 1 centre (7 elements)
   D. 5 planes, 5 axes and 1 centre
   Answer: C. Total 7 elements. (Key point: Orthorhombic symmetry)

3. Which set contains only monoclinic minerals?
   A. Galena, pyrite, halite
   B. Topaz, barite, olivine
   C. Quartz, calcite, dolomite
   D. Gypsum, orthoclase, mica
   Answer: D. Galena etc. are isometric; topaz etc. are orthorhombic; quartz etc. trigonal. (Key point: Monoclinic examples)

4. In the monoclinic system:
   A. alpha = gamma = 90 degrees and beta is not 90 degrees
   B. All angles are 90 degrees
   C. No angle is 90 degrees
   D. alpha = beta = gamma = 120 degrees
   Answer: A. Only beta departs from 90 degrees. (Key point: Monoclinic angles)

## Close variants set in other exams

- [Orthorhombic system; isometric mineral examples](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aorthorhombic-system-isometric-mineral-examples) — Engineering Geology I, chapter 3, asked 2×

## Related in Engineering Geology I

- [Symmetry elements, crystal systems, rock-forming minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Asymmetry-elements-crystal-systems-rockforming) — Engineering Geology I, chapter 3
- [Physical properties; orthorhombic symmetry elements](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-properties-orthorhombic-symmetry-elements) — Engineering Geology I, chapter 3
- [Isometric crystal system with symmetry elements](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aisometric-crystal-system-with-symmetry) — Engineering Geology I, chapter 3, asked 5×